Conventional parallel kinematics robots comprise a plurality of drive arms each connected, directly or via a gearbox, to a respective shaft of a servo motor at one end. At the opposite end each drive arm is connected to one or more rods, and the rods are further connected to an end effector. Between the drive arms and the rods, and between the rods and the end effector, respectively, there are joints with one to three degrees of freedom. The drive arms together with the rods and the joints form kinematic chains from the servo motors to the end effector for transmitting the rotating movement of the servo motors to a respective movement of an end effector. The servo motors and the respective drive arms work in parallel in the sense that manipulation of one drive arm does not affect the position of the remaining drive arms.
Each kinematic chain of a parallel kinematics robot provides the respective end effector with a degree of freedom. A delta robot is one well known type of parallel kinematics robot that typically comprises three drive arms and has three translational degrees of freedom. Each drive arm is connected to an end effector via two rods having a ball joint at each end. The drive arms rotate about respective servo motor axes, the servo motors being arranged symmetrically such that their axes intersect at 60 degrees angles. U.S. Pat. No. 7,188,544 discloses one type of a delta robot comprising three drive arms. Delta robots can also comprise four or more drive arms.
In many applications it is desirable to provide the end effector also with rotational degrees of freedom such that the end effector can change its orientation. US2014/0060234A1 discloses a parallel kinematics robot where the end effector has one rotational degree of freedom, and US2012/0060637A1 discloses a parallel kinematics robot where the end effector has three rotational degrees of freedom. In US2014/0060234A1 an additional actuator (which can be a servo motor) is arranged between two rods that are a part of a kinematic chain between a servo motor and the end effector. In US2012/0060637A1 three additional servo motors are arranged at a base of the robot such that they are immobile in relation to the three servo motors responsible for the translational movements of the end effector.
A drawback with the solution of US2014/0060234A1 is that the additional actuator adds to the weight of the kinematic chain it is attached to, and consequently the servo motor responsible for the movements of that kinematic chain needs to be dimensioned bigger or cannot move as fast as the case would be without the additional weight. A drawback with the solution of US2012/0060637A1 is that the work area of the robot is strongly limited by the largest allowed inclination of the transmission members transmitting the driving force from the additional servo motors to the end effector. Between the transmission members and the end effector there are namely cardan type universal joints that stop working properly as the bend angles of the joints become too large.
There is a desire to provide a parallel kinematics robot where the aforementioned drawbacks are mitigated.